1. Field of the Invention
This invention relates to the optically active benzene derivatives represented by the general formula (I): ##STR2## (wherein X represents --COO--, --OCO--, CH.sub.2 O-- or --OCH.sub.2 --; A represents a hydrogen atom, a halogen atom or an alkyl or alkoxyl group having 1 to 20 carbon atoms; R represents an alkyl group having 1 to 20 carbon atoms; l and m each represents a number of 1 or 2, n represents a number of 0 or 1, but when is 0 the sum of l and m is 3 or less; and * indicates asymmetric carbon atom), a process for producing the same, liquid-crystalline substances containing said derivatives as active ingredient, and optical switching elements utilizing the liquid crystal compositions containing said liquid-crystalline substances.
The term "liquid-crystalline substances" is used in this specification to refer to the liquid-crystalline substances of the broad sense, including those which may not have been confirmed to take a liquid crystal phase per se but can be utilized effectively as a liquid crystal composition.
2. Prior Art
Image display devices using liquid crystal are now practically used in various fields. Twinsted nematic type liquid crystal display is known as one of these display systems. This display system has the advantages that its power consumption is low and it is soft to the eye because of the light-receiving type (the display panel itself is not luminous). Display by this system, however, is not always satisfactory in the aspect of response speed.
As a system which is capable of high speed response, a display device utilizing the optical switching performance of ferroelectric liquid crystal has been proposed (Applied Physics Letters, 36, 899 (1980)) and is attracting attention.
In view of its molecular configuration, ferroelectric liquid crystal is considered to belong to the type of liquid crystal having chiral smectic C phase (hereinafter referred to as S.sub.C * phase) or chiral smectic H phase (S.sub.H * phase). With its high speed response characteristics, such ferroelectric liquid crystal is expected to find its use not only for display devices such as liquid crystal televisions but also as materials for electronic elements such as optical printer head, photo-Fourier transformation element, etc.
Such known liquid crystal compounds, however, had problems in practical use such as poor stability and were also unsatisfactory in response characteristics, etc., because of small spontaneous polarization. Also, even those compounds which show a high tendency of spontaneous polarization had problem in stability as they possessed halogen atoms in the molecular, and thus they could not be accepted as practical liquid crystal compounds.
In view of the above, the present inventors have made studies for developing a liquid crystal compound which can be widely applied to various types of display systems such as mentioned above, and as a result they found out novel optically active benzene derivatives and achieved the present invention.